1. Field of the Invention
This invention relates to an electronic flash device equipped with an electronic flash tube which is energized by discharge current of a main capacitor to emit light, and more particularly to such circuitry which includes means for controlling the amount of light emitted by the flash device.
2. Description of the Prior Art
FIG. 1 shows part of the circuitry of a prior art electronic flash device, in which integrating capacitor C and variable resistor R constitute a time constant circuit to which electric power starts to be applied from an energizing circuit (not shown), simultaneously with the commencement of emission of light of an electronic tube, whereby charging of the capacitor C is initiated. A base of transistor Q is connected to the junction between integrating capacitor C and variable resistor R. Then when the base potential thereof reaches a given reference level, a given time after the commencement of charging of integrating capacitor C, transistor Q is turned from a non-conducting condition to a conducting condition, with the result that a voltage is suddenly applied at terminal A. When the voltage is applied to terminal A, then a light-emission-interrupting circuit (not shown) is operated, thereby interrupting the feeding of a current from capacitor C1 to an electronic flash tube, so that the emission of light from the electronic flash tube is stopped. Variable resistor R is adjustable to vary its resistance value in accordance with a guide number setting. That is, when the guide number is set to a smaller value, then the resistance value of resistor R is reduced, hence the current flowing through the resistor is increased, and then integrating capacitor C is charged more quickly, so that the emission of light may be stopped before the termination of discharge of the main capacitor.
Assume that the aforesaid prior art electronic flash device is coupled with a camera of the type having an automatic wind-up mechanism which enables continuous shootings, and that with such a camera, flash photographs are carried out successively in series with guide number being kept constant. Upon the first photograph, with the electric power source switch for the flash device closed beforehand, then the main capacitor is charged sufficiently, so that sufficient light for the set guide number may be emitted from an electronic flash tube. The charging current for the integrating capacitor C is determined according to the set guide number so that the light-emitting time for an electronic flash tube may be constant for the constant guide number, whereas the quantity of light emitted per unit time from the electronic flash tube varies depending the charged condition of the main capacitor. Consequently, in case the charging of the capacitor is delayed due to consumption of the battery, then in the photographs following the first there is a successive lessening of the total quantity of light from the electronic flash tube, thus resulting in insufficient exposure of the film. This is particularly true in the case of a direct or series control type electronic flash device, wherein the emission of light of the electronic flash tube may be interrupted due to the interruption of discharge of the main capacitor. In other words, when the photography is repeated with an insufficient charging condition of the main capacitor, then the residual charges remaining in the main capacitor gradually decreases, so that in the successive photographs, the total quantity of light from the electronic flash tube is reduced per photograph, thus resulting in a difference in exposure level among pictures. In case a single picture is to be taken, even if a picture is taken in an under-exposure condition, it is of no consequence because of the absence of other pictures to be compared therewith. In contrast thereto, in case of successive photography, wherein the same photographic object is taken successively in a plurality of pictures, the difference in exposure condition between the first picture and the successive ones can be easily recognized and such difference gives the pictures a bad impression.
FIGS. 2 (a), (b), (c) and (d) show the charging condition of the aforesaid integrating capacitor and the light-emitting condition of the electronic flash tube. FIG. 2 (a) refers to a variation in charge current for an integrating capacitor with respect to time, FIG. 2 (b) refers to a variation in charge voltage for an integrating capacitor with respect to time, FIG. 2 (c) refers to a variation in voltage of the main capacitor at discharging condition with respect to time, and FIG. 2 (d) refers to a variation in intensity of light emitted from the electronic flash tube, wherein, throughout the figures, solid lines represent the case where the main capacitor is charged to a sufficient level, and broken lines represent the case where the main capacitor is charged insufficiently. As can be seen from these figures, the charge voltage for the integrating capacitor C reaches a reference level VS a given time after an initiation of flash firing, in accordance with the set guide number, irrespective of the charge voltage for the main capacitor, with the result that the light-emitting duration of an electronic flash tube may be maintained constant depending only on the set guide number, while the quantity of light per unit time variess, thus resulting in a difference in total light quantity to be emitted.
To cope with this, there has been proposed an attempt to measure directly the light emitted from the flash tube with a light receiving element facing the tube, and to interrupt the light emission of the electronic flash tube when an integration of the quantity of light measured by the light receiving element reaches a given level. However, this attempt involves difficulty in making uniform the characteristic of the light receiving elements, and requires the provision of a diaphragm or ND filter so as to limit the intensity of light incident on the light receiving element, with the accompanying complicated construction.
As an alternative, a device may be conceived in which a resistor is connected in series with an electronic flash tube in a discharge path for a main capacitor, so that a voltage drop across the resistor upon discharge of the main capacitor is detected to control the light-emitting time of the electronic flash tube in accordance with the detected value. This device, however, still involves a problem of a lower light-emitting efficiency of an electronic flash tube, due to energy loss in the resistor.